Geophysical prospecting



April 4, 1944. F. w. LEE

I GEOPHYSICAL PROSPECTING Filed May '7, 1940 2 Sheets-Sheet 2 Fig 7Feder/'Ck W Zee //2 Ven for Patented Apr. 4, 1944 UNITED STATES PATENTvOFFICE 1s claims. (ci. 11s-isz) (Granted under the act of March 3,1883, as

amended April 30, 1928; 370 0. G. 757) The invention disclosed hereinmay be used by or for the Government of the United States withoutpayment of any royalty therefor.

'I'his invention relates to geophysical prospecting or surveying andaims generally to improve the same.

In particular this invention enables the determination of the contour,shape, position and/or composition of geologic bodies, hereafterreferred to as geologic structure, and more particularly provides amethod of and means for making such determinations by electricalresistivity, electrical dielectric and magnetic permeabilitymeasurements obtained at the surface of the ground and other accessiblepoints, such as shafts, stopes, drifts and drill holes. The inventiondifferentiates ground which may be electrically isotropic or anisotropicin regard to its resistivity, dielectric constant, and magneticpermeability, hereafter referred to as electrical properties. Byanisotropic properties of geological structure is meant that thematerial has different resistivities and electrical properties indifferent directions. This invention concerns itself in dening geologicstructure by separating and using the directional reslstivities andelectrical properties as further to be described.

In the accompanying drawings, exemplifying the principles andapplication of the present invention:

Fig. 1 is a perspective view showing a suitable ground coniiguration ofcurrent electrodes Ci-Cz, preferably equally spaced at opposite sides ofa station point Po, and .showing a suitable location of potentialelectrodes relative thereto;

' Fig. 2 is a diagrammatic representation of the current flow andtransverse potential values under isotropic conditions or underanisotropic conditions symmetrical longitudinally and transversely ofthe line of centers of current electrodes;

Fig. 3 is a similar diagram, for anisotropic conditions symmetrical onlyabout the plane transverse to the center line of current electrodes;

Figs. 4, 5 and 6 are similar diagrams for different conditions;

Fig. '7 is a circuit layout for application to the ground conflguration;and

Fig. 8 is a three dimensional diagrammatic illustration showingsubterranean anisotropic conditions.

A simple case where an isotropic geologic body overlies an anisotropicone is found in glaciated districts Where the glacial till covers thehard A rock outcrops las for example in northern Michigan. Suchhard-rock outcrops are often banded in composition and tilted, whichmakes the ground anisotropic in character. deflne'these diterentiallyconducting bands constitutes the nature of this invention, even whencovered with other isotropic or anisotropic material.v

It is well known in the art that by applying a potential to spacedconnections on the ground an indication may be obtained which relates toor is a function of the electrical resistivity and other electricalconstants of the ground. See F. W. Lee, U. S. Patent No. 1,951,760,granted March 20, 1934.

In explaining the present invention, it is desirable to lirst considerthe electrical resistivity which for convenience may be divided into twocomponent parts, one being called the normal resistivity as lwould existin an isotropic medium, the other the transverse resistivity as wouldbe' caused by the non-isotropic portion of the medium. For simplicity ofmeasurement and mathematical analysis the two resistivity components areseparated in space by a right-angle relationship,-such that Wherep=apparent ground resistivity; pn=normal component of the apparentground resistivity; pt=transver component of the apparent resistivity;z'=\/-l (i is the operator square root of m. inus one). Thus theabsolute value of p=.\/pn'-|p:2 and the direction is arc tan Q While itis possible to compute pn and pt between spaced potential and currentcontacts on the ground, this invention discloses a, method fordiierentiating the two in such a way that each may be measured andinterpreted separately. Furthermore it, divides the ground in such amanner that .the change of symmetry in the transverse resistivitiesdiscloses the changes of ground structure. In the above mentioned casewhere the ground is covered with glacial till the sand and gravel is ofan isotropic character for which pn would have a value and pt thetransverse resistivity would be zero.

Referring to Fig. 1, showing a suitable ground configuration, current ofsuitable nature is applied through the ground electrodes CiCz,preferably equally spaced at opposite sides of the central or stationelectrode Po, as shown. 'I'he dotted line electrodes PiPz, which areconsidered The ability to as located on a line with CiCz, may be used todetermine the normal resistivity component, as explained in my abovementioned patent and my copending application entitled sistivity orimpedisivity measuring, Serial No. 200,948, iiled April 8, 1938 patentedMarch 31., 1942, Patent No. 2,277,707. To determine the .transverseresistivity particularly "contemplated in the present invention, andnewly coordinated thereby with the normal resistivity to give com' plexdata as set forth, the new coordination of transversely positionedelectrodes Pa, ,P4, preferably equally spaced on opposite sides of thestationvpoint Po and at right angles to lthe line of centers of thecurrent electrodes CiCz, is used. This arrangement divides the ground bytwo partitioning planes, passing vertically into the ground through CiCzand through PaPr, respectively. Measurements on both sides of thevertical plane CrCz, by means, for example, of potential electrodesP3P4, in accordance with the present invention, enables determination ofthe anisotropic character changes on each side of this plane, asindicated by the transverse resistivity of the geologic structure.

Now referring to Fig. 2, representing isotropic conditions, it will beseen that with the electrode configuration .shown the transverse currentfor small values of Po-Pa and Ply-P4 owing in the direction P3-P4 wouldbe zero, since the field would be symmetrical about the Ps-P4 axis aswell as about the Pi--Pa axis. With measurements of increasedseparations of C1--Cz, Po-Pr, Po-Pz, Po-Pa, Po-P4 appreciable parts ofthe current will flow deeper and deeper in the terrain and current willeventually begin to ow through the underlying bed rock. If the bed rockis anisotropic, then potential will begin to appear between Po-Ps andPo-Pi and by noting the separation when this occurs it is possible tocompute the depth of the isotropic overburden.

In practicing the present invention the point Po, Fig. 1, is called astation point at which measurements are made at successive depths byincreasing the distances between the other points of the congurationwhile maintaining the relative symmetry. The station points arepreferably located at spaced distances along a line of traverse. Suchspaced distances are preferably chosen so as to not omit, any portion ofground and preferably so as preliminarily to eliminate the infrequentlyfound positions at which the transverse observations become zero. Suchpositions may be found when the material AB, which has anisotropicelectrical properties, passes perpendicularly between Ci-Cz at Pu ofFig. l or when it passes parallel to CiCz as shown in Fig. 3. 'I'he morefrequently found and preliminariiy more useful positions are shown inFigs. 4, 5 and 6.

In Fig. 4 the transverse components of potential are equal but inopposite direction. In Fig. 5 the transverse components of potential areequal and in the same direction and in Fig. 6 they are unequal dependingupon the position of AE in reference to symmetry about the line C1C2 andthe distance from Po. After a conducting materialv AB has been identiedas for example in Fig. 6, or Fig. 4, the point P is adjusted until anarrangement similar to Figure is found, in which P0P: and P0P4 are,nearly equal in value indicating that Po is substantially electricallycentered over the vein (AB). By then varying the azimuth of C102relative to AB, the two positions of zero indication when AB liesparallel or normal to Ci-Cz (giving a diagram slmrilarto Fig. 2) serveto 'further delineate the structure.

Fig. 8 shows a hypothetical cross-section of the earth in whichthematerial AB sought for is dipping to the left. The isotropicoverburden D covers this material. Adjacent to AB is material C whichdiffers materially from the conductivity of AB. The stream lilies ofcurrent E are indicated on the top of the ground on which measurementsare made at the spaced electrical configuration of distances a and a.

Referring again to Figs. 4 lt will be -observed that the direction ofvectors will reverselfA-B'lies on the other side of Po, or if thepolarities of CiCz are reversed; from Fig. 5, it will be apparent thatif the position of strike is displaced upwardly toward P3, parallel tothe direction shown, the vector directions of Pa, P4 will remain thesame, but their values will become unequal; and from Figs. 4, 5 and 6 itis apparent that when, and only when both P3 and P4 are on the same sideof the position of strike, will the vector directions be opposed.

Furthermore',"if the configuration, relative to anomaly AB, lig.V 5, isturned about P0 as a center, the values between PoPs and PuPi willgradually decrease, being zero when AB coincides with eitherrPaP4 orC1Cz, and the vector directions shown willreverse from quadrant toquadrant, the vector values being equal to one another so long as Po iselectrically centered over AB.

From the foregoing analysis it will be appreciated that the similarity,or opposition, of the vector directions relative to each other, theirrelative values and their absolute directions and values, in variouscombinations, will evidence denite position of strike and dip.

In the foregoing only the ohmic resistivity or direct. currentimpedisivity was considered; in the alternating current case asexplained in my above mentioned copending application, the impedisivityinvolves not only a real part corresponding to ohmic resistivity butalso an imaginary part com..

, posed of a time vector operator which is a function of frequency,dielectric constant, and permeability of the material. A similaranalysis is made in the alternating current case, using transverseimpedisivity instead of resistivity, the phase-shift, or time vectoroperator, due to the dielectric and magnetic permeability constants ofthe structure, being determined by means .of a phase shifter asexplained in my said copending application.

Y In the method of making ground observations exemplied in Fig. 7, bothdirect current and alternating current may be applied to the same groundcontacts and the alternating characteristics of the ground compared tothe direct current permitting vdiierentiations of asymmetric materialbased on direct measurements between PoPi and P0P: and transversemeasurements between PoPa and P0P4.

Thus the vector magnitude, in either the direct current or fluctuatingcurrent case, indicates and enables determination of the position of theunderlying strata and aids in identifying the composition of thematerial; and the phase shift, in the fluctuating current case, givesfurther evidence of the composition of the materials of the structure.It will be appreciated by those skilled in the art that a uctuatingcurrent may be an alternating current of simple sinusoidal or othercontinuous or discontinuous form, alone, or suvperposed on a directcurrent component, as may 4, 5 and 6: from Fig;`

- fluctuating current cases a best suit the conditions of use, althoughfor most generally sinusoidally varied alternating current is preferred.

To obtain a complete identification of isotropic and anisotropicmaterial the potential electrodes P1 and Pn,

with my Patent No. 1,951,760-are employed as well as the transverselypositioned electrodes Pa-Po--P4, the symmetry of which entirelyeliminates normal impedisivlty, thus giving evidence oi' the transverseresistivity uninuenced by the isotropic factors,

By using both Pi-Pn and Pa-P4 with the same Ci--Po-4-Cz configuration,it is possible for the rst time to obtain complete geophysical evidencegiving knowledge of the character and depth of isotropic overburden andcontact with and position of anisotropic stratified and bandedunderlying material.

To summarize, by establishing a configuration exemplified in Figs. 1 and8; (1) with relatively short distances values a, a1, conditions atrelatively shallow depths can be determined. These will usually beisotropic, as where a homogeneous overburden overlies the rockformations, zero values being determined for the transverseimpedisivities between Po and Pa and between Po and P4, the character ofisotropic material being indicated by values appearing between P1 and Poand P2 and Po.

(2) By increasing the distances a and al, greater depth of survey isaccomplished, and when repetitions result in the appearance of valuesbetween Po and Pa or between Po and P4, or both, then it is indicatedthat contact has been established with anisotropic material at the depthof survey functionally corresponding to the a, a1 distances in use.

(3) By then varying the azimuthal direction of line Ci-Cz the directionof strike may be determined as above explained.

Y (4) At this same time the-relative values appearing between Po and P1,and between Po and l Pz evidence the direction andangle of dip, if

gentle enough to lie within the range of both observations, withoutrepeating the set-up of the configuration at another location, as wouldbe necessary if only Po, Pa, P4 were used.

(5) For steep dips, not within the range of Po-Pi, and Po-Pzobservations, an accurate indication can be obtained by moving theconfiguration longitudinally in the direction of dip indicated by item3, above, and repeating items 1 and 2 in the new location to determinethe depth of the anisotropic structure thereat.

(6) During items 1 and 2, and before the orientation of item 3 isreached, the phase-shift factor of `impedisivity determined by applyingfiuctuating current to the configuration, serves to give evidence of thecharacter of the anisotropic material.

(7) Also, at any time evidence lof the asymmetric locations ofsink-holes, faults, dikes, etc., with reference to the partitioningplane along Ci-Czmay be determined by the directions and values of thepotentials appearing between Pu and Pa or Po and P4 or both; and at thesame time evidence of the locations thereof relative to the plane alongP3-P4 may be determined by the relative values of potentials appearingbetween Po and P1, or Po and Pz. Thus the anomalies are seggregated intoquadrants with only a single set up of the configuration.

(8) By repeated set-ups and measuring of the preferably with electrodePo-giving data as to the normal impedisivity, in accordance change invalues and direction of the potentials of the transverse system atvarious values of a and a1, it is possible to determine the change ofstrike through geological unconformities. 'Ihus if one system of beddinghaving a definite `strike and dip overlies another system of beddinghaving a different strike and dip, or stratified in another direction,this may be determined.

While it will be apparent from the foregoing description vthat variousarrangements may be used for applying and measuring the configurationcurrents and potentials, Fig. '7 shows for purposes of illustration, themanner of employing the arrangement disclosed in my copendingapplication for "Electrical resistivity or impedisivity measuring, Ser.No. for this purpose. Y.

As shown in Fig. 7, the resistivity or impedislvity measuring devicecomprises threeunits, corresponding to the three units of my saidapplication disclosure. The upper, or current-input unit is connected toapply a predetermined direct or fluctuating current to the currentelectrodes C102. The lower, or potential measuring unit is arranged tobe connected to measure either the normal impedisivity between PiPn andPaPo, or the transverse impedisivity between PaPo and P4Po, preferablyby suitable switching means, as the triple-pole double throw switchshown; and, as fully discussed in my above-mentioned application,preferably makes these measurements by balancing` out the picked uppotentials to prevent flow of current through the potential electrodes,

200,948, filed April 8, 1938,

thus avoiding the disturbances of the electric field distribution in theearth which are always produced when currents are drawn from thepotential electrodes, and enabling the ground resistivitles orimpedisivities to be measured by the relative magnitudes and signs ofthe balancing potentials. The center unit, as in my copendingapplication, is the calibrating unit enabling field calibration of thepotential measuring unit directly from the current input unit.

This device, as more fully explained in said co pending application,Where its novelty is claimed per se', enables determination of theimpedisivities both in the normal and in the transverse directions asregards quantity and sign, which is evidenced as direction in the directcurrent case, and as phase shift in the fluctuating current case, and isto be so interpreted in the appended claims.

Furthermore, where certain of the claims, for brevity, employ the term"Lee configuration" this term designates the configuration shown inFigs. 1 and 8 of the drawings; Where they employ the term directimpedisivity, this designates the values appearing between Po-l-Pi andbetween Pri-Pz; while the term transverse impedisivity designates thevalues appearing between Po-Ps and Pia-P4; the term depth determiningdimensions designates the size of the configuration as determined by thedimensions a, a1; and the term point of origin or origin designates thestation point Po. l

While I have described preferred embodiments of the several cooperatingfeatures of my invention, it is to be understood that these embodimentsare but illustrative, and not restrictive of my invention.

I claim as my invention:

1. A method in geophysical surveying, comprising the steps of (a)establishing a flow of electric current through the earth between twopoints, (b) measuring potentials from a third point intermediate thefirst two points to other points'on a line transverse to the lineconnecting the first two points, (c) determining the relative magnitudeand sign of the transversely measured potentials which reflect and thusindicate the presence and location of striise of subterranean strata,and (d) repeating steps a, b and c in varis ous orientations of currentow near the position at which a subterranean strike is indicated, untilequality of magnitude, with opposite sign if magnitude is other thanzero, establishes accurately the direction of strike.

2. A method comprising steps a, t and c oi claim l, in which therelation of current to electrode spacing is maintained in apredetermined ratio throughout a seris of determinations.

3. A method comprising steps a, b and c of claim l, which includesfurther determining slope of the underlying strata by taking furtherpotential measurements from the intermediate point to further points onthe line of the irst two points, and in which the relation of current toelectrode spacing is maintained in a predetermined ratio throughout aseries of determinations.

4. In geophysical surveying, the step of determining factors of positionof an underlying structure by applying current to the terrain at theends of a base line and measuring the impedisivity of the terraintransverse to the approximate center eof said base line.

5. In geophysical surveyingl the step of determining factors of positionof an underlying structure by applying current to the. terrain at theends of a base line and measuring the impedisivity of the terrainsymmetrically transverse to the approximate center of said base line.

6. In geophysical surveying, the step.of determining factors of positionof an underlying structure by applying current to the terrain at theends of a base line and measuring the impedisiv ity of the terrain atright angles to the approximate center of said base line.

7. The method of determining the presence and azimuthal direction ofstrike of a subter ranean strata in geophysical surveying whichcomprises selecting a. point of origin for the survey, causing a currentto flow through the earth from points on a substantially straight linepassing through the point of origin, and measuring the transverseimpedisivity between the origin and points spaced transversely onopposite sides thereof, and determining from the said measuredimpedisivities the approximate position and direction of strike of thesubterranean strata..

8. The method of determining the presence and azimuthal direction ofstrike of a subterranean strata in geophysical surveying which comprisesselecting a point of origin for the survey, causing a current to nowthrough the earth from points on a substantially'straght line passingthrough the point of vorigin and equally spaced therefrom, and measuringthe transverse impedisivity between the originV and points equallyspaced transversely on opposite sides thereof on a line at right anglesto the first straight line, and determining from the said measuredimpedisivities the approximate position and Idirection of strike of thesubterranean strata.

9. A method according to claim 8, which fury ther includes determiningaccuratelyV the said azimuthal direction of strike by repeating thesteps of claim 8 at locations on lines substantially normal to theapproximate direction' of strike until substantially equal and oppositetransverse impedisivity measurements are obtained.

the

audace i0. The method of determining, with a sevenelectrodeconfiguration applied to the terrain, the depth of anisotropic materialin geophysical surveyng; which comprises employing one electrode as apoint of origin, locating four other electrodes along a base linepassing through said point of origin, connecting two of said otherelectrodes to apply electrical energy to said terrain and the other twoto pick up potentials from said terrain which reflect and thus indicatethe impedisivity of the terrain embraced in said. configuration,locating two electrodes at opposite sides of said base line along a.line transverse to said base line and connecting said last twoelectrodes to pick up potentials from said terrain which reflect andthus indicate the transverse impedisivity of the terrain embraced 1nsaid configuration, and repeatedly performing said steps with differentdepth determining dimensions of the configuration applied to the terrainto determine the depth at which indications of transverse impedisivityappear.

ll. The method of determining, with a flveelectrode connguration appliedto the terrain, the direction of strike of anisotropic material ingeophysical surveying; which comprises employing one electrode as apoint of origin, locating two other electrodes along a baseline passingthrough said point of origin, connecting said other electrodes to applyelectrical energy to said terrain, locating two electrodes at oppositesides of said base line along a line transverse to said base line andconnecting said last two electrodes to pick up potentials from saidterrain which reect and thus indicate the transverse impedisivity of theterrain embraced in said configuration, and varying the azimuthalorientation of said conguration about its point of origin whilemaintaining depth determining dimensions of said conguration such thatindications of transverse impedisivity are obtained.

l2. The method of determining the direction and dip of gently slopinganisotropic material in geophysical surveying; which comprisesestablishing by the method of claim 11 the azimuthal orientation of theconfiguration therein deiined corresponding to symmetry with respect tothe direction of strike, and picking up potentials aligned with the baseline of said conguration which reflect and determine the directimpedisivities of the coniiguration so oriented as an indication of thedirection and angle of dip.

13. The improved conguration for geophysical surveying comprising apoint of origin Po, current electrodes C102 positioned on a first linepassing through said point of origin and on opposite sides of andsubstantially equidistant from said point of origin, and means fordetermining transverse impedisivities comprising potential electrodesP3P.; positioned at opposite sides of the point of origin on a linepassing through said point of origin and transverse to said first line.

14. The improved configuration for geophysical surveying comprising a.point of origin P0, current electrodes CiCz positioned on a first linepassing through said point of origin, and means for determining directand transverse impedisivities comprising potential electrodes P1P:positioned at opposite sides of the point of origin on said rst line andother potential electrodes PaP4 positioned at opposite sides of thepoint of origin on a line passing through said point of origin andtransverse to said rst line.

15. A method in geophysical surveying. which comprises establishing aflow of electric current through the earth between two points, pickingup potentials from a third point intermediate the rst two to otherpoints on a line transverse to the line connecting the first two points,balancing out the picked up potentials to prevent disturbance of theelectric ield established in the earth by the flow of current betweensaid ilrst two points, and determining the relative magnitude and signof the balancing potentials which reilect and thus indicate the presenceand location of strike of subterranean strata.

16.I A method in geophysical surveying, which comprises establishingrent through the earth between two points, picking up potentials from athird point intermediate the rst two to other points on a. linetransverse to the line connecting the rst two points, balancing out thepicked up potentials to prevent disturbance of the electric ieldestablished in the earth by the flow of current between said iirst twopoints. and determining the relative magmtude and phase of the balancingpotentials which reect and thus indicate the presence and location ofstrike of subterranean strata.

17. A method in geophysical surveying, which comprises establishing aiiow of direct current through the earth between two points, picking uppotentials from a lthird point intermediate the rst two to other pointson a line transverse to the line connecting the nrst two points.balancing out the picked up potentials to prevent disturbance pf theelectric iieid established in the earth by the flow of current betweensaid rst two points, and determining the 'relative magnitude anddirection of the balancing potentials which reiect and thus indicate thepresence and location of strike of subterranean strata.

a ilow of alternating cur- 18. In geophysical surveying, the methodwhich comprises applying to the terrain a seven-electrode coniigurationcomprising a station electrode Po, two current electrodes C1 C2equidistant from the station electrode Po and lying on a base linepassing through the station electrode P0, two normal-potentialelectrodes P1 and P2 equidistant from the station electrode Po and alsolying on the said base line, and two transverse potential electrodes P3P4 lying on a line at right angles to said base line and passingthroughsaid station electrode, orienting said conguration in such directionthat no potential diierence exists between the transverse potentialsobtained at the transverse potential measuring electrodes P3 and P4,thereby positioning the current base line CiCz of the configuration inalignment with the direction of slope of underlying strata, andmeasuring the potential differences between the normalpotentialmeasuring electrode pairs P0P; and which reect and thus indicate, insuch orientation, the true direction of slope of the underlying strata.

